The present invention relates to body implantable prostheses, and more particularly to stents and stent grafts insertable into blood vessels and other body lumens and radially expandable against surrounding tissue for fixation.
A variety of treatment and diagnostic procedures involve devices intralumenally implanted into the body of the patient. Among these devices are stents, such as disclosed in U.S. Pat. No. 4,655,771 (Wallsten). The Wallsten prostheses or stents are tubular, braided structures formed of helically wound thread elements. The stents are deployed using a delivery catheter such as disclosed in U.S. Pat. No. 5,027,377 (Burton, et al.). With the stent positioned at the intended treatment site, an outer tube of the delivery catheter is withdrawn, allowing the stent to radially expand into a substantially conforming surface contact with a blood vessel wall or other lumen-defining tissue.
An alternative stent construction features plastically deformable strands or members, usually formed of a ductile metal. Examples of such stents are shown in U.S. Pat. No. 4,776,337 (Palmaz), and U.S. Pat. No. 5,716,396 (Williams, Jr.). Such stents do not require outer tubes or other features to maintain them in a reduced-radius state during delivery. However, radial expansion at the treatment site requires an expandable balloon or other feature for enlarging the stent.
Regardless of whether stents are self-expanding or plastically deformable, they characteristically have an open mesh or open frame construction, or otherwise are formed with multiple openings to facilitate radial enlargements and reductions, and to allow tissue in-growth. Further, either type of stent can be used to support a substantially fluid impermeable material, frequently but not necessarily elastic, to provide a stent graft for shunting blood or other body fluids past a weakened or damaged area, e.g. an aneurysm.
Stents and stent grafts of either type must be constructed in view of two competing considerations: flexibility for axial bending, and radial rigidity or strength. Axial bending, i.e. bending of the stent about any number of transverse (radial) axes along its length, is necessary for translumenal delivery through curved, even serpentine vessels and other passages. Radial stiffness and strength are needed after delivery and radial enlargement, to maintain the lumen open against any radial forces from surrounding tissue. In stent constructions featuring helically wound strands, there also is a need to balance radial strength and rigidity with the desire to minimize the degree of axial shortening that accompanies any radial enlargement. Radial strength is enhanced by a large braid angle, i.e. a relatively steep pitch of the strand helix relative to the longitudinal stent axis about which the helical strands are wound. Conversely, a reduced braid angle or pitch involves less axial shortening for a given amount of radial enlargement. Joining all of the intersections or crossings of strands has been suggested to enhance resiliency in self-expanding stents. This, however, increases the resistance to axial bending, making the device difficult to maneuver through tortuous passages, and imparting a high axial stiffness after radial enlargement.
The aforementioned '396 patent discloses a structure intended to provide radial support while allowing axial bending flexibility. In particular, a helically wound strand of malleable material has a repeating pattern of undulations or bendable segments, each with a semi-circular portion and two straight leg portions. Adjacent turns of the helix are joined by at least one weld joining two of the semi-circular portions. Aside from the complexity of shaping the strand to produce the undulations, an undesirable structural feature resides in the welding of successive turns, in that structural integrity for resisting axial tension depends entirely on the welds.
Other problems, more particular to plastically deformable stents, concern the tendency to recoil, i.e. radially contract, after their radial enlargement. This creates the need to over-enlarge the stent or stent graft, to insure its fixation. Also, a radial compression of the plastically deformable stent, typically about an expandable balloon at the distal tip of a catheter, is necessary prior to deployment. Such compression must be done with care, to insure a uniform radial contraction, and to avoid irregularities or distortions in the stent structure.
Therefore, it is an object of the present invention to provide a body insertable prosthesis in which helically wound strands are selectively joined to one another at selected crossings of the strands, to favorably influence the combination of radial stiffness and axial bending flexibility.
Another object is to provide a stent or stent graft formed of helically wound strands with a desirable combination of a lower braid angle and enhanced radial stiffness.
A further object is to provide a process for manufacturing a medical device incorporating a plastically deformable stent or stent graft, that provides for a more uniform radial compression of the stent or stent graft about its deployment carrier, to reduce the risk of irregularities or distortions during such compression.
Yet another object is to provide a plastically deformable stent or stent graft that can be radially enlarged with relative ease, and after deployment has a high degree of resistance to radial compression and recoil.